This invention relates to a photosensitive body for electrophotography and more particularly to a photosensitive body with a photoconductive layer composed mainly of amorphous silicon.
Basic requirements for a photosensitive body to be practical and useful in electrophotography include high resistivity and high photosensitivity. As materials having these required characteristics, the following two types of materials have been most commonly utilized, the dispersed resin type obtained by dispersing cadmium sulfide powder in an organic resin and the amorphous type such as amorphous selenium (a-Se) and amorphous arsenic selenide (a-As.sub.2 Se.sub.3). These materials are not satisfactory for reasons of their environmental effects, however, and development of their substitute materials is desired. In recent years, amorphous silicon is seriously considered as such a substitute material.
Not only is amorphous silicon environmentally harmless but its photosensitivity is high and it is also a very hard material. Because of these favorable characteristics, amorphous silicon is expected to be usable as a superior material for a photosensitive body. If amorphous silicon is used alone, however, the value of its resistivity is not high enough to retain electric charges during an electrophotographic process. In order that amorphous silicon be used in a photosensitive body for electrophotography, therefore, something must be done such that a high charge voltage can be maintained without adversely affecting its high photosensitivity.
One of the attempts to this end was to increase the resistance of an amorphous silicon layer itself which serves as the photosensitive body. In order to effectively make use of the superior photoconductive characteristics of amorphous silicon (such as strong optical absorption, relatively large electron and pole mobilities and long-wavelength sensitivity), however, it is more desirable to provide a blocking layer with a large energy band gap on the surface (and the base member) to prevent the charge from escaping rather than to increase the resistance of the photoconductive layer itself so as to improve its capability to be charged. Such a surface layer of the type having a large energy band gap may be considered indispensable not only for keeping charges but for establishing surface stability as a surface protective film which not only protects the photosensitive body from severe bombardments of corona ions during an electrophotographic process but also reduces the variations in the characteristics due to changes in environmental conditions such as temperature and humidity. To serve as a surface protective film, surface layers with a larger energy band gap are naturally more desirable.
As described above, it is generally to be considered desirable to provide a surface layer with a large energy band gap not only for keeping charges but also from the point of view of surface protection. If a layer with a large energy band gap is formed immediately on the surface of amorphous silicon forming an electroconductive layer, however, there appear characteristics which are undesirable for a photosensitive body for electrophotography. One of such undesirable characteristics is mechanical instability. If a surface layer with a large energy gap is formed on an amorphous silicon photoconductive layer, stable adhesion cannot be obtained between the surface layer and the photoconductive layer because of their difference in coefficient of thermal expansion.
Electrically undesirable characteristics also appear if a surface layer with a large energy band gap is directly formed on a photoconductive layer. If the surface area of a photosensitive body which is preliminarily charged is exposed to light in an electrophotographic process, charges of polarity opposite to those on this surface layer are generated in the photoconductive layer and these generated charges move through the photoconductive layer, cancelling the surface charges electrostatically. If the energy band gap of the surface layer is large as explained above, however, the gap becomes very large at the boundary between them and charges cannot move smoothly. As a result, these charges become accumulated near the boundary surface between the surface layer and the photoconductive layer and their effect appears as a residual potential. The residual potential is not a desirable thing. If the residual potential increases, it can cause deterioration of the characteristics of the photosensitive body. Moreover, the residual potential frequently induces movement of accumulated carriers in transverse directions, causing fogginess in the image.
In short, although a surface layer with a large energy band gap is indispensable from the point of view of charge retention and surface protection, it causes both mechanical and electrical problems, and no photosensitive body of amorphous silicon satisfactory for electrophotography has been available yet.
Moreover, conventional photosensitive bodies for electrophotography with a photoconductive layer having amorphous silicon as the principal constituent can be charged only positively or negatively. Thus, the carrier mobility in the photoconductive layer can be large only for holes in the case of a photosensitive body intended to be charged positively and only for electrons in the case of a photosensitive body intended to be carged negatively. U.S. Pat. No. 4,613,556 issued Sep. 23, 1986 to Mort et al. describes an amorphous silicon photosensitive body for electrophotography which can be charged in both positive and negative modes. This photosensitive body is a device with a hydrogenated amorphous silicon photosensitive layer and a charge transporting layer of plasma deposited amorphous silicon oxide. The charge transport channels in the silicon oxide layer or the charge transport manifold accepts photo-excited carriers in the hydrogenated amorphous silicon to provide the ambipolar nature, that is, the photosensitivity in both the positive and negative charging modes. In other words, there has been no photosensitive body available for electrophotography with sufficiently large mobilities for both holes and electrons. As a result, there could be no photosensitive body available with superior sensitivity both when charged positively and negatively.